1. Field of Invention
The present invention generally relates to a method and apparatus for current measurement, and more particularly to a method and apparatus for current measurement using Hall sensors without iron cores.
2. Description of Prior Art
In power systems, safety and reliability are the most important considerations. In order to meet safety and reliability requirements of the power systems, appropriate current transformers are used to measure currents for metering and fault protection.
A traditional current transformer is an apparatus for current measurement. The traditional transformer using the coil magnetic effect has an iron core. Therefore it has the problems of direct current and alternating current saturations, and remanent magnetization. All of the problems mentioned above are caused by the effect of hysteresis. The problem of direct current saturation caused by the effect of hysteresis may affect the current measurement of the current transformer most severely.
When faults occur in power systems, fault currents usually contain large direct offsets, and thus, may cause iron cores of current transformers to be saturated. This may cause distortion of current waveforms in secondary windings, resulting in false responses in current detection and protective systems. These problems seriously affect the safety and reliability of power systems.
Since traditional current transformers have these problems, including direct current and alternating current saturations, and remanent magnetization, caused by iron cores, several technologies for decreasing the effects caused by iron cores have been proposed. The technologies include air-gap current transformers, linear coupler current transformers, electro-optics current transformers, and linear current transformers (i.e Rogowski coil transformers.).
However, in practical applications, only the linear current transformer is used, and the others are not used due to their disadvantages and low practicalities. When a large transient current flows in a power system, the linear current transformer can perform better than the traditional current transformer. Nevertheless, the cost of the linear current transformer is high, and thus, causes limited usage of the linear transformer. Moreover, the bulky volume of the traditional current transformer further limits the development of an intelligent power protection system. Therefore, the research of the current transformer is still one of the important issues in power industry.
According to properties of the electromagnetic field in a power cable when an alternating current flows in this cable, based on Ampere's circuital law, a magnetic field is produced outside the insulator, and it changes with the waveform of current. For sensing the variation of the magnetic field and decreasing the bulky volume, Hall sensors have been researched for replacing traditional current transformers. Based on Lorentz force, Hall sensors have quick response times, and can measure a wide range of magnetic fields. Moreover, the related compensations can be integrated into Hall sensors to help them show steady performances and have high sensitivity. Therefore, Hall sensors can be used to replace traditional current transformers.
At present, Hall-effect transducers include two parts: iron cores and auxiliary circuits, either open-loop circuits or closed-loop circuits. These transducers are mainly used to measure small currents with large bandwidths. They still contain iron cores, because iron cores not only can transform the magnetic field into electricity but also can block the interference of the ambient magnetic fields. Furthermore, with iron cores, measurements of Hall current transformers will not be affected by the geometric shape of the measured object, and the magnetic field thereof can be concentrated.
However the Hall current transformers with iron cores still have the problems of direct current and alternating current saturations, and the volumes of them are also almost the same as those of traditional current transformers.